Despite being one of the most abundant elements forming the rocky planets and small bodies in our Solar System, not to mention an essential component of the air all living things breathe, measuring the amount of oxygen in rocks and minerals is downright tricky. Oxygen atoms are bound within the structure of minerals, and if removed, they immediately form a gas and float away or combine with nearby hydrogen to form water. In fact, most standard chemical analyses only measure the abundances of the other elements, and then estimate the amount of oxygen needed to get an answer that makes sense. However, this only works when you already know what materials are present in your sample, which is not always the case for dust on the surface of asteroids and the Moon.
As mentioned in a previous blog post, the ancient surfaces of asteroids have been “reddened” by the effects of space weathering. This does not mean the asteroids are actually red, but rather that the light reflected from their surfaces contains a higher than expected amount of red, infrared, and other long wavelength components. It has been previously observed that space-weathered surface dust from the Moon and asteroids contains tiny metallic iron beads in the upper 100 nanometers of the surface of the dust grains. In addition, computer calculations of light scattering from rock surfaces can reproduce the “reddening” effect of space weathering by adding a small amount of nanophase iron metal beads to their models. But how did these iron beads form? And do the iron beads form more rapidly in some minerals than in others?
Iron metal only contains iron atoms; there is no oxygen in its crystal structure. On space-weathered surfaces, it must have formed from decomposition of other iron-bearing minerals, most of which contain many oxygen atoms. On a simplified level, then, the nanophase iron metal beads are produced by destroying iron-oxygen chemical bonds until there are no oxygen atoms left. This type of chemical reaction is called “reduction”, since the number of chemical bonds to oxygen atoms are decreased. This is the same process that occurs when using rust remover to restore the metallic shine to rusted metal surfaces, or when smelting iron ore at high temperature to form molten iron metal. The opposite process, “oxidation”, or the addition of chemical bonds with oxygen atoms, is much more common in our everyday lives, such as the browning of an apple after biting into it, or the formation of rust in the first place. But the key to understanding extreme reduction of iron-bearing minerals due to space weathering is finding a reliable way to directly measure the number of oxygen atoms bonded to iron atoms.
One of the ways the RIS4E team can measure the number of iron-oxygen chemical bonds, and therefore estimate the amount of oxygen in a sample, is through a technique called electron energy loss spectroscopy (EELS). This technique is performed using a powerful electron microscope by measuring the energy of the electrons after they have passed through a sample. Most electrons won’t have lost any energy, but a small number that pass by atoms in the sample will transmit some of their energy to any chemical bonds that are connected to those atoms, including iron-oxygen chemical bonds. On the EELS detector, this appears as an intensity spike (indicating a large number of electrons that have lost energy) at the unique energy of the bond. In the case of iron atoms, the EELS spectrum has peaks at 708.5 electron volts (eV) for a 1:1 ratio of iron and oxygen or at 709.5 eV for a 2:3 ratio. Chemists use an alternate way of communicating these ratios, based on the electronic charge the iron atom would have if it were free by itself. The 1:1 ratio of iron to oxygen is known as iron(II) or Fe2+, while the 2:3 ratio is written as iron(III) or Fe3+. Oxidation reactions create more iron-oxygen bonds, generating more Fe3+, while reduction reactions remove oxygen, generating more Fe2+. Most rock-forming minerals, including the common minerals found in asteroids, have a mix of Fe2+ and Fe3+ atoms, somewhere between these two oxidation ratios, and so both peaks are present in EELS analyses.
Also, space weathering of each kind of mineral may proceed at a different rate, or occur by a different mechanism. One of the ultimate goals of the RIS4E team is to build a better understanding of oxidation-reduction reactions in iron-bearing minerals during space weathering. We are beginning this work by performing high-resolution measurements of a series of glass samples synthesized with different element abundances to match the major igneous rock types (for example, basalt, andesite, etc.) that could be present on asteroids or the Moon. These glass samples will serve as baseline data that we can compare with all subsequent analyses of various minerals and artificially space-weathered samples. Team members Drs. Rhonda Stroud, Bradley De Gregorio, and Kate Burgess are using the new picometer resolution Nion UltraSTEM electron microscope at the Naval Research Laboratory, one of only a handful of such instruments in the world. The primary feature of this electron microscope is an aberration-corrector lens module to remove spherical aberrations that are created by the main objective lens of the microscope, and are present in all electron microscopes. The result of this aberration correction is an ultra-small electron beam, capable of picometer image resolution, and enhanced spectral resolution for EELS measurements. The myriad of crates containing the pieces of the UltraSTEM were delivered to the lab in July of 2013, and thanks to the dedication and hard work of Nion engineers, a picometer resolution image was acquired a week later. With a complicated, custom-built instrument like this, it took nearly six months to tune and calibrate the electron microscope in order to achieve the ultimate resolution of the EELS detector. We have obtained preliminary data from the glass standards [see Figure], but I’ll let Kate tell you all about those results. Much work still needs to be done to optimize the measurement process to obtain robust, repeatable results. Then, we can move on to common mineral standards, such as pyroxene and olivine, and, eventually, we will perform these same measurements on actual lunar and asteroidal dust!
For more information see:
K. D. Burgess, R. M. Stroud, B. T. De Gregorio, M. D. Dyar, and M. C. McCanta (2015) Measurement of Fe oxidation state using aberration-corrected scanning transmission electron microscopy. 46th Lunar and Planetary Science Conference, Lunar and Planetary Institute, The Woodlands, TX, Abstract #1965.